The mixing together of two or more polymers has attracted interest as a means of arriving at new property combinations without the need to snythesize novel structures. In most cases, when two polymers are mixed, the components tend to segregate into separate phases, forming a non-adhering, heterogeneous mixture that exhibits inferior overall properties. Only occasionally do polymers blend to form a single homogeneous amorphous phase.
The term polymer blend refers to an intimate mixture of two or more polymers. Blends of polymers are often described as miscible or compatible, and the terms are sometimes considered to be equivalent. The term miscible will be used herein to describe a mixture of two or more polymers that form a single-phase solution (solid or liquid) on a molecular scale within the amorphous phase. When one or both of the polymer blend components is capable of forming both a crystalline and an amorphous phase, then the term miscible refers only to the amorphous phase in which the separate components are capable of mixing on the molecular level.
Miscibility can be achieved by selecting components that interact with one another in an attractive mode (e.g. which mix exothermically). Several methods can be used to determine miscibility in polymer blends. For example, when a film is prepared from a miscible blend, it is usually optically clear, while immiscible blend films are usually opaque. However, this criterion is not useful when one of the blend components is crystallizable.
The most commonly used criterion for miscibility is the existence of a single glass transition temperature for a given miscible blend. This parameter is relatively easy to measure for amorphous systems, and rapid if a technique such as differential scanning calorimetry is used. Greater sensitivity, especially useful for semicrystalline blends, can be obtained when dynamic mechanical methods are employed to measure the glass transition temperature. As the relative proportion of components changes, a smooth change between the glass transition temperatures for the pure blend components and the glass transition temperatures for the various blends will be observed over the miscible range for the blends.